Communication and power cable

ABSTRACT

A cable containing an optical fiber for conducting communication signals and electrical conductors for conducting power. The optical fiber is centrally located. An inner layer includes electrical conductors and an outer layer includes electrical conductors interspaced with strength members. The interspaced strength members are in electrical contact with the electrical conductors and lower the loop resistance so that a power source may be used at one end of the cable and the equipment to be powered at the other without requiring an excessively large voltage at the power source. In one embodiment, the strength members are formed of steel and are interspaced such that there is one strength member per two electrical conductors. The cable is enclosed in a smooth and continuous stainless steel outer sheath.

This is a divisional of copending application Ser. No. 07/540,573 filedon Jun. 15, 1990 (now U.S. Pat. No. 5,140,319).

BACKGROUND

The invention is related generally to the remote viewing of well boresand other limited access areas, and more particularly, to an apparatushaving a camera for remotely viewing the condition of such limitedaccess areas.

There has long been a need in the field of well boring to visuallyexamine the bore hole to examine geological formations or for otherreasons. Where those well bores have casings and fittings, there is acontinuing need to inspect the casings and fittings for corrosion andother conditions. By means of visual inspection, the locations of leaksand points of infiltration can be more easily identified.

One existing method of accomplishing this is to insert an instrumentprobe containing a camera, such as a closed circuit television camera,in the bore hole and move it through the area to be inspected.Typically, a compact, rugged instrument containing the camera and alight source is provided as an instrument probe. A cable is attachedbetween the instrument probe and the surface station to communicatecamera signals to the surface from the instrument probe. In onetechnique, the communication link comprises optical fibers. Where thebore is vertical, gravity can be used to pull the instrument probethrough the bore hole. The support cable contains strength members whichpreserve the integrity of the cable as it and the instrument probe arebeing pulled from the bore hole.

Optical fibers offer certain advantages in communication systems. Theyare relatively immune to electromagnetic interference, they haverelatively low cable weight, they have a large bandwidth, high frequencyvideo signals can be transmitted over long lengths of cable with minimumattenuation and they have lower cost. Thus they are desirable in aremote logging instrument probe such as that used to view bore holes.However, optical fibers are sensitive to point stresses and bending. Thefiber may impart significant attenuation to its conducted signal whenbent. The cable system of a well logging instrument is repeatably pulledaround at least one sheave and wound on and off a winch drum as it islowered into and lifted out of bore holes. The cable must withstandrepeated bending and tensions of thousands of pounds. Stretching thecable can stretch the optical fibers thereby increasing their stress andaggravating their attenuation. High pressures and high temperatures inthe well holes may assist moisture in invading the cable and the opticalfibers. Moisture invading the optical fiber through micro-cracks canincrease its attenuation and reduce its strength. Thus, the cableconnecting the instrument probe with the remote controller must protectthe optical fibers as well as be strong enough to withstand repeatedbending about sheaves and a winch drum, withstand stretching forces andthe high temperatures and pressures in the bore hole.

A logging instrument probe for well holes must be rugged to withstandthe sometimes harsh conditions encountered in typical operation. Forexample, hydrostatic well pressures in excess of 4.2×10⁶ kilograms persquare meter (6,000 pounds per square inch) and ambient walltemperatures of up to and above 190° C. (375° F.) are not uncommon. Thepoint where the support cable enters the instrument probe must haveeffective seals to deflect such high pressure/temperature fluids fromentering. The main purpose of such seals is to protect the camera, theoptical fibers and the electrical connections from the fluids present inthe bore hole. Such seals have been difficult to manufacture on arepeatable basis and make the instrument probe very expensive as well asunreliable. Thus, it would be desirable to provide an effective sealarrangement which can be manufactured on a repeatable basis and whichuses standard parts readily available thereby lowering the cost of theseal.

Another common condition in bore holes is turbidity in the form ofgases, mud, oil, and other fluids under high pressure. In priortechniques, high intensity lighting such as that provided by quartzlamps or halogen lamps, is provided to give bright light in the visiblerange for use with conventional television cameras. Depending upon theamount of turbidity, higher intensity light from the lamps may berequired to provide clear images.

In order to provide power for such lamps and for the camera and otherequipment, well bore inspection instruments carry a self-contained powersupply, typically battery packs. In addition to adding weight and bulkto the instrument probe, these battery packs have a limited life whichis directly dependent upon the intensity of the lights. As an example,many battery packs of the size which can fit in a bore hole logginginstrument probe can provide power for only 3 to 31/2 hours when usedwith halogen lamps. Upon dissipation of the stored energy, the batterypacks must be removed and replaced with charged batteries or a chargingprocess must occur which may take many hours. This usually requires thatthe instrument probe be removed from the well hole, dis-assembled andre-assembled. This can be a time consuming process which subjects thesupport cable of the instrument probe to the added stresses of beingpulled over the sheave and wound on the drum an additional time.

To the inventor's knowledge, remotely locating a power source has upuntil this time not been practical in an application where a smalldiameter optical fiber is employed in the support cable having theextremely long lengths required for deep well holes. The power sourcehas been included in the instrument probe itself. This co-location ofpower source with instrument probe was caused by the long lengths ofcable required for use in deep well bores. In prior cables, the steelstrength members of the cable were used as part of the electricallyconductive path. Typically, the strength members are made of steel whichhas a relatively high specific resistance, and therefore a low resistiveloop circuit was not available to carry the required current at anacceptable working voltage. The diameter of the strength members wasincreased to result in less resistance; however, even with thistechnique, the loop resistance was relatively high and the increasedsize of the strength members resulted in a large cable with theassociated disadvantages of high weight and bulk. Additionally, suchstrength members were used to form the outer surface of the cable, thusmaking it rough and more difficult to handle.

Hence, it would be desirable to provide an improved power arrangement sothat the power source for the well logging instrument probe could beremotely located from the probe, such as at the surface, yetimpracticably large voltages would not be required in order to get therequired power over the typically long distances between the surface andthe probe. Additionally, it would be desirable to provide a smallersupport cable and a support cable having a smooth outer surface tofacilitate handling, yet one having the required strength. Additionally,those concerned with instrument probes for use in viewing bore holes andcasing have recognized the need for an improved sealing system to keepfluids from entering the probe which can be manufactured on repeatableand economical bases. The invention fulfills those needs.

SUMMARY OF THE INVENTION

Briefly and in general terms, the present invention provides a videologging inspection system with an instrument probe having improvedsealing and a remotely located power source. In one embodiment, theinstrument probe comprises a closed circuit television camera and anoptical fiber or fibers are used to conduct the camera signals to thesurface for remote viewing. A light source is placed forward of thecamera for illuminating the walls of the bore hole, casing and fittings.Three mounting bars are used to firmly extend the light source forwardof the camera lens and protect the electrical power cables powering thelights. The length of the bars is selected in dependence upon the innerdiameter of the bore hole or casing.

The power source for both the camera and the light source is located atthe surface. Power is conducted to the components in the instrumentprobe through a multi-conductor support cable having the optical fiber,the electrical conductors and the strength members all enclosed in asmooth outer sheath which in one embodiment is made of stainless steel.The outside sheath is thin enough so that it may be routinely pulledaround sheaves and rolled on the winch drum for transport yet is strongenough to protect the inner conductors and strength members. The supportcable has the optical fiber located in the center and a buffer materialsurrounding the optical fiber. Next is a layer of inner power conductorswhich are surrounded by a layer of insulation. Next is a layer ofstrength members alternating with outer electrical conductors.Surrounding this layer is insulation and surrounding this and all layersis the outer sheath of the cable.

In order to prevent fluids from entering the instrument probe along thesupport cable entry point, a multiple seal arrangement has beenprovided. In one embodiment, the sealing arrangement comprises threeseals. The first seal comprises flexible material such as multiple"O-rings" compressed around the support cable at the end of theinstrument probe where the support cable enters. The second seal isformed in conjunction with the breakaway connection. A strength tube isplaced over a section of the cable, the outer sheath removed thereafterand the strength members and outer electrical conductors are bent backover the tube. A fluid resistant, adhesive caulking material such asepoxy is applied to the bent-back strength members and outer conductorstrands. A bulkhead tube, which is rigidly connected to the bulkhead, isthen mounted over the bent-back strength members and conductors. Enoughepoxy is used to fill the gaps between the strands and to also fill thebulkhead tube around the cable to the bulkhead. The epoxy not only holdsthe strands in place but also provides a fluid seal. The bulkhead tubemakes electrical contact with the bent back electrical conductors toestablish an electrical path.

The third seal is formed at the bulkhead of the instrument probe. Athreaded compression coupler is screwed at one end into the bulkhead.The threads used for the bulkhead connection are in one embodiment NPTpipe threads, thus providing a fluid seal. The remainder of the supportcable (less the already bent-back members) proceeds through the bulkheadtube and through the bulkhead. A metal sheath also containing a fluidresistant, adhesive caulking material is mounted over this portion ofthe support cable and enters the compression coupler at its otherthreaded end. A compression fitting slides over the metal sheath and athreaded compression nut is screwed onto the second threaded end of thecompression connector to compress the compression fitting onto the metalsheath thus forming a fluid seal at the bulkhead. The epoxy forms aninternal fluid seal at the bulkhead while the compression fitting formsan external seal.

Because the power source is located at the surface, no battery packs areneeded in the instrument probe and it can be used for much greater timeperiods for observation of the conditions of the bore hole, the casingand the fittings. A support cable provides not only the optical fibersupport and strength members but also the power transmission meansnecessary to operate the camera and lights and other equipment in theinstrument probe. The triple seal of the cable in the instrument probeprovided by the invention assures protection of the instrument probeequipment and is easily and relatively inexpensively provided.

These and other objects and advantages of the invention will becomeapparent from the following more detailed description when taken inconjunction with the accompanying drawings of illustrative embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall block diagram of a well logging system according tothe present invention;

FIG. 2 is a side view of an instrument probe which is in place in a wellbore showing a part of the support cable, cable head, camera section andthe light section;

FIG. 3 is a partial cross-sectional side view of part of the camerasection of the probe showing the camera, cabling, power distributionsection, fiber optic transmitter section, lens and mounting points ofthe support legs used to hold the light section in front of the lens ofthe camera;

FIG. 4 is a partial cross-sectional view of the light section of theinstrument probe showing a halogen lamp and the legs and mountings;

FIG. 5 is a cross-sectional view of a support cable in accordance withthe principles of the invention;

FIG. 6 is a partial cross-sectional view of the cable head showing thethree seals of the support cable and the instrument probe in accordancewith the invention; and

FIG. 7 is an exploded view of the third seal in accordance with theprinciples of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, like reference numerals will be used torefer to like or corresponding elements in the different figures of thedrawings. Referring now to the drawings with more particularity, in FIG.1 there is shown a well logging system 10 comprising a well boreinstrument probe 12 which has been lowered into a well bore 14, asupport cable 16, a sheave 18, a rotatable winch drum 20, a surfacecontroller 22, a controller enclosure 23 and a transportable platform 24which in this case is a skid unit. One end of the support cable 16 iswound on the drum 20 which supports the cable 16 for raising andlowering it into the bore 14 as the drum is rotated. The surfacecontroller 22 controls the operation of the winch and the probe 12 andreceives and processes information provided by the probe 12. The controlenclosure 23 may include a recorder, such as a video tape recorder, forrecording the information provided by the probe 12.

Referring now to FIG. 2, an instrument probe 12 is shown in greaterdetail and is presently disposed in a bore 14. The support cable 16 isconnected to the probe 12. The probe 12 has three sections, a cable head25, a camera head 26 and a light head 28. The light head 28 is attachedto the camera head 26 through three legs 30, two of which are shown.Legs of different lengths may be used depending upon the inner diameterof the bore 14 or casing. The larger the inner diameter, the longer thelegs should be so as to not interfere with the camera viewing angle.

Referring now to FIG. 3, the camera head 26 part of the instrument probe12 is presented. Section 32 of the cable 16 is coupled to the opticaltransmitter section 34. At this component, electrical signals from thecamera representing camera images are converted into optical signals andcoupled to an optical fiber disposed within the cable. The optical fiberis used for the transmission of the images to the surface.Electrical/optical converters are well known in the art as well ascouplers for coupling the converter 34 to the optical fiber and nofurther details are given herein.

The next section in the camera head 26 is the electrical section 36. Theelectrical power brought into the instrument probe 12 by the cable 16 iscoupled to this section where the electrical power is converted into thevoltages needed by the camera, the light and the electrical/opticalconverter. For example, the voltage supplied by the cable 16 may be 100Vdc while the camera operates on 12 Vdc and the light on 50 Vdc. Theelectrical/optical converter may require 12 Vdc. Such converter boardsare well known in the art, for example, Model SWA175-4300 by Power-One,Inc., Cammarillo, Calif.

The next section in the camera head 26 is the camera 38 itself. In oneembodiment, the camera was a charge coupled device (CCD) type televisioncamera which is capable of providing high speed, high resolution imagesin relatively dim light. One camera found to be usable in an embodimentis the CCD Video Camera Module having a model number of XC 37 made bySony Corporation. Coupled to the camera is a lens 40 which in oneembodiment was a fisheye lens, and a quartz window 42. The window 42seals the camera head 26 at its bottom end and protects the lens 40against high pressure/high temperature fluids which may exist in thewell bore. Its angle is selected so as to not obstruct the viewing angleof the lens 40. Also shown in FIG. 3 is part of the legs 30 which arewelded to the camera head 33 in this embodiment and which hold the lighthead 44 in position in front of the lens 40. The electrical conductors48 are separately routed through the legs to the light head. In theembodiment shown, three legs were used although only two legs are shownin this figure.

Referring now to FIG. 4, the light head 28 is shown having a halogenlamp 46. The light head 28 is attached to the camera section by legs 30which are also welded into the light head 28 in this embodiment. Thelength of the legs is selected based on the inner diameter of the boreor casing. Where the camera must see further because of a larger innerdiameter of the casing, the legs 30 are made longer so that the lighthead will not obstruct the view of the lens. Where the inner diameter issmall, the legs may be shorter so that more light is placed within thecamera viewing angle. Thus, several different light heads with varyinglengths of legs may be required. Electrical power conductors 48 whichprovide electrical power to the light traverse one or more of the legs30. Other light sources may be used such as incandescent lamps.Additionally, light other than visible light may be used, for example,infrared and ultraviolet.

In accordance with the invention, power for the instrument probe 12resides at the surface at the controller 22. In one embodiment, thepower source at the controller 22 is transmitted to the support cable 16via slip rings at the drum 20 in accordance with techniques known tothose skilled in the art.

In order to conduct the necessary power over the long distances commonlyexperienced by video logging systems, a multi-layer cable is provided.Referring now to FIG. 5, a cross-sectional view of a cable in accordancewith the invention is presented. Disposed at the center is the opticalfiber 50 and immediately around it is a buffer layer 52. Although only asingle optical fiber is shown in this embodiment, multiple fibers may beused to comprise a fiber optic cable. The buffer layer 52 providesmechanical isolation. Surrounding the buffer layer 52 and coaxialtherewith is an inner layer of electrically conductive strands 54 whichin one embodiment are formed of copper. These strands form a powerconductor which conducts power from the controller 22 to the instrumentprobe components. Surrounding the power conductor 54 is a layer ofinsulation 56 and surrounding the insulation is a layer comprisingstrength member strands 58 alternating with outer, electricallyconductive strands 60. In this embodiment, one outer conductive strand60 is interspaced with two strength member strands 58.

To balance the resistance of the power conductors 54 with the resistanceof the strength members 58 which also serve as one leg of the resistiveloop, ten outer electrically conductive strands which in one embodimentare formed of copper, are alternately interspaced in the total of thirtystrands in the layer. The power conductor resistance when formed ofcopper strands is 6.1 ohms per 305 meters (1000 feet). The twenty steeland the ten copper resistive loop strands have a resistance of 18.1 ohmsper 305 m (1000 ft). Thus, the loop resistance is 24.2 ohms per 305 m(1000 ft). Because of this arrangement of alternately interspaced copperand steel strands, the loop resistance is lowered. The diameter of thestrength members then may be selected to satisfy only strength concernsrather than both strength and electrical conductivity concerns. It hasbeen found that in a cable in accordance with the invention, thestrength members are significantly smaller thus resulting in a muchsmaller and lighter support cable yet one with loop resistance lowenough so that a manageable voltage may be used at the surface.

In one embodiment, the optical fiber used was a 50/125/245 multi-modefiber with a buffer formed of Hytrel which is available from Dupont inWilmington, Del. The optical fiber had a 0.050 mm core with 0.125 mmcladding. The Hytrel buffer was 0.18 mm (0.007 in.) in averagethickness. The power conductors were 0.38 mm (0.015 in.) in diameter andwere formed of copper HDBC, the insulator was 0.48 mm (0.019 in.) inthickness, and each strength member and resistive loop conductor was0.25 mm (0.010 in.) in diameter. The insulation was compounded Hytreland the strength strands were formed of improved plow steel.

Surrounding all of the above members is an outer sheath 62 which in oneembodiment, is made of stainless steel sheet having a thickness of 0.20mm (0.008 in.). In one embodiment, the stainless steel sheet was formedinto a tube shape and welded lengthwise to form the outer sheath. Thestainless steel is strong and fluid resistive thus protecting theinternal components. It was found that forming the outer sheath ofstainless steel provides a smooth outer surface thus facilitating itsmovement in pressure sealing glands and around sheaves and drums.Because it is relatively thin, it can be rolled on the sheaves and drumsas necessary.

Because of the commonly-experienced high pressures of fluids found inbores, the cable entry point into the instrument probe 12 must besufficiently sealed to protect against the entry of extraneous fluids.In accordance with the invention, a series of three fluid seals isprovided. Each seal is formed of commonly available parts thus beingeconomical to provide.

Referring now to FIG. 6, mounted at the cable head 25 is the first seal64. It comprises a compressive type seal using flexible materials suchas ordinary "O-rings." In this embodiment, three O-rings are used andare compressed onto the outer cable sheath 62 as it enters theinstrument probe. Two of these O-rings 66 comprise metal washers whichhave rubber seals inserted within them. These are commonly called Parker"Thread Seals" or "Stat-O-Seals" and are available from Parker Seals,Culver City, Calif. The third O-ring 68 is a conventional O-ringinserted between the two Parker thread seals.

Referring now to FIGS. 6 and 7 for a description of the second seal, theouter sheath 62 is terminated and a length of tubing 71 is mounted on aselected length of the outer sheath 62 adjacent the termination point.The steel strength members 58 and outer conductors strands 60 are foldedback over the tubing 71. The optical fiber, buffer, power conductorstrands and insulation are not folded back. The folded back strands arethen coated liberally with a fluid resistant adhesive caulking material73 such as that sold as Tri-con #230 epoxy made by Tri-Con, Inc. of 55North Street, Medford, Mass. After coating with epoxy, an outer bulkheadtube 72 is placed over the folded back strands. As is well known in theart, the number of strands folded back and their lengths determine thebreak-away force required for the cable to be separated from theinstrument probe 12. By selecting the number and length of the bent backstrength members, the force may be set so that if the instrument probe12 should become stuck in a bore or casing for some reason, the cablemay be pulled out of the instrument probe and then the instrument probecan be recovered by other means. In the event that a reduced breakawayforce is required, some of the strength members may be cut but notfolded back.

The bulkhead tube 72 makes electrical contact with the bent back membersthus putting the bulkhead tube 72 and bulkhead 74, to which the tube isjoined, in the electrical return path. Additionally, in the embodimentshown, the length of cable covered by the bulkhead tubing 72 is alsocoated with the caulking 73 so that all parts of the cable covered bythe bulkhead tubing 72 are coated. Upon setting, the adhesive caulking73 will hold the support cable in position inside the bulkhead tubing aswell as provide a fluid seal. In the event that greater electricalconductivity is required, an electrically conductive epoxy may be used.

Referring now more particularly to FIG. 7, welded to the other end ofthe bulkhead tube 72 is a bulkhead 74. As shown, it has fluid seals 75on its outer surface. A threaded bore is formed through the bulkhead 74for placement of the third seal 76. Mounted in the threaded bore andcomprising part of the third seal is a compression coupler 78. Thecoupler 78 has NPT pipe threads formed on its end which is connected tothe bulkhead thus forming a fluid seal. The other end of the coupler isshaped to accept a compression fitting and a compression nut forcompressing the fitting. In the third seal, a compression sheath 80 alsohaving a fluid resistant, adhesive caulking material placed inside ismounted over the remainder of the support cable which proceeds throughthe bulkhead. In one embodiment, a compression sheath 80 formed of brasstubing having a 3.2 mm (0.125 in.) outer diameter by 2.3 mm (0.090 in.)inner diameter. Placed over the compression sheath 80 is a compressionfitting 82 and placed over the fitting 82 is a compression nut 84. Asthe compression nut 84 is turned onto the coupler 78, the compressionfitting 82 is forced into closer contact with the compression sheath 80thereby forming a fluid tight seal. The compression fitting 82, nut 84and coupler 78 are all standard "plumbing" parts which are commonlyavailable.

Thus by use of the three seals, the connection of the support cable withthe instrument probe 12 is made fluid tight for use in high pressureenvironments. Additionally, because all of the seals are made ofstandard components, the three seals are economical to provide andmanufacturing on a repeatable basis is facilitated.

Past the third seal, a contact 86 is formed on the compression sheath80. Because the compression sheath 80 is in physical and electricalcontact with other third seal parts which are in electrical contact withthe bulkhead 74 which is in electrical contact with the bulkhead tube 72which contacts the folded back steel strength members 58 and outerconductor strands 60, a potential is made available at the contact 86 asindicated by the terminal symbol. Past this contact, the powerconductors 54 are folded back to make a second potential available. Pastthis point, the optical fiber 50 continues.

Thus, in accordance with the invention, a new and useful inspectionsystem is provided having the power source for the instrument probelocated at the surface rather than in the instrument probe itself. Thedesign of the support cable results in lower loop resistance thusallowing a smaller cable and a more manageable supply voltage at thesurface. Additionally, a smooth yet strong outer sheath on the cablemakes it easier to handle yet it provides protection for the internalcable components. The fluid seals are relatively simple, easy toimplement, lower in cost and can be manufactured on a repeatable basis.

It will be apparent from the foregoing that, while particular forms ofthe invention have been illustrated and described, various modificationscan be made without departing from the spirit and scope of theinvention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

What is claimed is:
 1. A communication and power cable comprising:anoptical fiber; an inner layer of electrical conductors surrounding theoptical fiber; an outer layer of electrical conductors surrounding theinner layer of electrical conductors; an insulator disposed between theinner layer of electrical conductors and the outer layer of electricalconductors, thereby separating said layers; and strength member strandsdisposed in one of said layers which are in electrical contact with theelectrical conductors of said layer in which they are disposed, thestrength member strands having a tensile strength substantially greaterthan the tensile strength of the electrical conductors of the layer inwhich they are disposed, but having an electrical conductivitysubstantially less than said electrical conductors.
 2. The cable ofclaim 1 further comprising a continuous, metallic, tubular, sealed andsmooth outer sheath which surrounds the optical fiber, inner, and theouter layers.
 3. The cable of claim 2 wherein the outer sheath comprisessteel.
 4. A communication and power cable comprising:an optical fiber;an inner layer of electrical conductors surrounding the optical fiber;an outer layer of electrical conductors surrounding the inner layer ofelectrical conductors, the second layer being separated from the firstlayer; an insulator disposed between the first and second layers suchthat none of the electrical conductors of the first layer are inelectrical contact with the electrical conductors of the second layer;and a plurality of strength members located in the second layer suchthat they are interspaced with and in electrical contact with theelectrical conductors of the second layer, the strength member having atensile strength substantially greater than the tensile strength of theouter layer electrical conductors, but having an electrical conductivitysubstantially less than the electrical conductors.
 5. The cable of claim4 wherein one strength member alternates with two electrical conductorsin the outer layer.
 6. The cable of claim 4 further comprising acontinuous, metallic, tubular, sealed and smooth outer sheath whichsurrounds the optical fiber, the inner, and the outer layers.
 7. Thecable of claim 6 wherein the outer sheath comprises stainless steel. 8.A communication and power cable comprising:an optical fiber; an innerlayer comprising a first plurality of electrical conductors surroundingthe optical fiber; an outer layer comprising a plurality of electricallyconductive strength members surrounding the inner layer and separatedfrom said inner layer such that there is no electrical contact betweenthe inner layer and the outer layer, and a plurality of electricalconductors interspaced with and in electrical contact with the strengthmembers, the strength members having a tensile strength substantiallygreater than the tensile strength of the outer layer electricalconductors, but having an electrical conductivity substantially lessthan the electrical conductors, wherein the number of electricalconductors disposed in the outer layer is selected to achieve apredetermined resistance in the outer layer; and a continuous, sealedand smooth outer sheath which surrounds the optical fiber, the innerlayer, and the outer layer.
 9. The cable of claim 8 wherein one strengthmember alternates with two electrical conductors.
 10. The cable of claim8 wherein the outer sheath comprises stainless steel.